DC component in VFD transformers
Certain VFD topologies may generate direct current (DC) component on the grid side. There is a general concern about how such DC component affects the performance of the VFD transformer. In this post we discuss some basics of DC generation and its impact on transformers.
DC component in drive systems
The voltage and current waveform synthetized by VFDs is usually not perfectly sinusoidal. Despite the admirable progress in power quality there is still certain harmonic distortion. We usually talk about harmonics, i.e. frequency components that are multiples of the fundamental frequency. However, there may be small portion of sub-harmonics and eventually also a DC component.
Impact of DC component on transformer performance
Main concern about presence of DC component is transformer saturation. As known, transformer is designed to transform alternating current (AC) and voltage. When exposed to DC there is a unipolar magnetization. It increases the flux density which can lead to core saturation.
Saturated transformer is not able to provide its primary function which is the voltage transformation. Load side voltage drops and shows high distortion. The saturation is accompanied by increased magnetizing current. In an extreme case the magnetizing current is so high to cause an overcurrent trip. Other consequences of saturation are increased audible noise and higher losses.
How much DC is too much?
Transformer is able to withstand certain small amount of DC component. The reason for DC may not be linked to the VFD operation only but can result from grid unbalance or transient disturbances. What is the DC level that a “normal” transformer can tolerate and when is a special design required?
VFD topologies injecting DC component
Some topologies tend to inject more DC than others. There are two basic mechanisms of generation of DC component. In our full article we describe both of them. You will also learn other factors having impact on the magnitude of DC component and overall severity.
DC is not always DC
When talking about DC component we need to consider additional aspects such as duration (continuous or just temporary), real DC or a low frequency nature, constant or variable magnitude etc.
This post is just a little “teaser”. if you want to know more subscribe to our premium content or purchase our paper “DC component in VFD transformers: Origin, impact and mitigation”. It covers following parts:
- Mechanism of DC component generation
- Nature of DC component in drive applications
- VFD topologies that may inject DC component to the grid
- Severity of DC component for transformer design
- Effects of DC component on transformer design
- Mitigation methods and resilience
- Practical experience
Purchase the paper for CHF 11.90 and learn more about the subject.
